1. Field of the Invention
The present invention relates generally to a phase locked loop (PLL) for use with a local oscillator, and more particularly to a PLL which has provided with a loop gain control circuit and a frequency modulated (FM) feedback circuit.
2. Description of the Prior Art
In the art, the frequency of a local oscillator used in a receiver is synchronized in phase with the reference frequency of a reference oscillator, such as a quartz oscillator or the like. In order to vary the frequency of the local oscillator, a variable frequency divider is used, which is varied in response to the frequency of a received signal to provide a predetermined frequency band or range.
In general, the frequency stability of a PLL is determined by its loop gain, which is determined by the phase conversion gain of the phase comparator and the frequency conversion gain of a voltage controlled oscillator (VCO) in the PLL that uses a variable frequency divider so as to vary the frequency of a local oscillator, the loop gain thereof is varied by the frequency dividing ratio 1/N. It is known in the art that in order to cover a predetermined frequency band, the frequency dividing ratio increases with the frequency of the local oscillator and, hence, the loop gain of the PLL becomes lower than at low frequencies.
Further, the harmonic locking method in which no variable frequency divider is employed and the frequency of the local oscillator is phase-locked to harmonics of the reference oscillator is disclosed in the IEEE Spectrum March, 1971, page 42. In that case, the loop gain is also lowered at high frequencies of the local oscillator.
However, it has not been proposed heretofore to make the loop gain of a Pll substantially constant over the frequency band of the local oscillator.
In the art an FM feedback method has been known in which a negative feedback signal is supplied from the output of a frequency discriminator to a local oscillator to improve the sensitivity of a receiver. However, it has not been proposed previously to produce a receiver in which a PLL circuit and the FM feedback are combined to suppress the drift in the local oscillator and to improve the sensitivity of the receiver.